In recent years, an optical element made of glass is utilized in a wide range of applications such as a lens for a digital camera, an optical pickup lens for a DVD, a camera lens for a cell phone and a coupling lens for optical communication. As such an optical element made of glass, a molded glass article manufactured by pressure molding of a glass material by use of a molding die is widely utilized.
As one of such methods for manufacturing a molded glass article, known is a method in which a glass preform having a predetermined mass and form is prepared in advance and the glass preform is heated together with a molding die to be subjected to pressure molding (hereinafter, also referred to as “reheat press method”). In recent years, to reduce the manufacturing cost of a glass preform, study of a method, in which a molten glass droplet dropped from a dropping nozzle is received by a molding die and then cooled as it is to prepare a glass gob (a glass block), which is utilized as a glass preform, is in progress.
On the other hand, as another method for manufacturing a molded glass article, proposed is a method in which a dropped molten glass droplet is received by a lower mold and the glass droplet received is subjected to pressure molding by a lower mold and an upper mold to be a molded glass article (hereinafter, also referred to as “liquid droplet molding method”). This method is noticed because repeated heating and cooling of molding dies (a lower mold and an upper mold) is not required and a molded glass article is efficiently manufactured from a molten glass droplet.
As a molding die for manufacturing a glass gob or a molded glass article by such a method, known is a molding die in which a protective film containing chromium, which shows an excellent releasing property for glass, is formed on the surface of a mother material made of various ceramics or a super hard alloy (Japanese Laid-Open Patent application Publication No. H04-260619). Further, proposed is a method, in which oxygen is ion-injected into a protective film containing chromium and a heating treatment is provided in an atmosphere containing oxygen, to improve durability of the protective film (Japanese Laid-Open Patent application Publication No. 2000-327344).
In protective film containing chromium, chromium is oxidized by heating to form a layer of chromium oxide on the surface. Chromium oxide has very low standard free energy of formation (standard Gibbs' energy of formation) and is very stable to show low reactivity with glass, resulting in an excellent releasing property.
However, there was a problem that, when oxidation of protective film proceeds with long term use, the roughness of the surface is increased due to crystal growth of chromium oxide, and the surface roughness of a glass gob or a molded glass article having been manufactured may thus be increased. A method described in Japanese Laid-Open Patent application Publication No. 2000-327344 not only requires an ion-injection process which is complex and expensive but also shows an insufficient effect to restrain crystal growth of chromium oxide.